JP2009277735A - Method of manufacturing thermoelectric material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of manufacturing a thermoelectric material such that when the thermoelectric material containing Ba and/or Ga is manufactured, an outflow of Ba and/or Ga is suppressed and the composition is precisely controllable. <P>SOLUTION: The method of manufacturing the thermoelectric material includes a heat treatment process of heat-treating a principal member consisting of a thermoelectric material which contains at least one of Ba and Ga as a constituent element and whose absolute value of a Seebeck coefficient at room temperature is ≥20 μV/°K or a precursor to become the thermoelectric material through solid phase reaction while making the principal member and an auxiliary member made of a compound containing at least one of constituent elements coexist. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for manufacturing a thermoelectric material, and more particularly, to a method for manufacturing a thermoelectric material having Ba and / or Ga as constituent elements.

Thermoelectric conversion refers to the direct conversion of electrical energy into thermal energy for cooling and heating, and conversely, thermal energy into electrical energy using the Peltier effect and Seebeck effect. Thermoelectric conversion
(1) Do not discharge excess waste during energy conversion,
(2) Effective use of exhaust heat is possible.
(3) It can continuously generate power until the material deteriorates.
(4) No moving devices such as motors and turbines are required and maintenance is not required.
Therefore, it has been attracting attention as a highly efficient energy utilization technology.

As an index for evaluating the characteristics of a material capable of mutually converting thermal energy and electrical energy, ie, thermoelectric material, generally, a figure of merit Z (= S ² σ / κ, where S: Seebeck coefficient, σ: electric conduction Degree, κ: thermal conductivity), or a dimensionless figure of merit ZT expressed as a product of the figure of merit Z and the absolute temperature T indicating the value. Further, σS ² obtained by removing κ from the figure of merit Z is called an output factor, and is used as an index for electrical evaluation of the thermoelectric material.
Here, the Seebeck coefficient represents the magnitude of the electromotive force generated by the temperature difference of 1K. Thermoelectric materials each have their own Seebeck coefficient, and are broadly classified into those having a positive Seebeck coefficient (p-type) and those having a negative Seebeck coefficient (n-type).

The thermoelectric material is usually used in a state where a p-type thermoelectric material and an n-type thermoelectric material are joined. Such a junction pair is generally called a “thermoelectric element”. The performance index of the thermoelectric element depends on the performance index Z _{p of the p-} type thermoelectric material, the performance index Z _{n of} the n-type thermoelectric material, and the shape of the p-type and n-type thermoelectric materials, and the shape is optimized If you are the greater the Z _p and / or Z _n, the figure of merit of the thermoelectric element increases is known. Therefore, in order to obtain a high figure of merit thermoelectric device, the figure of merit Z _p, be used with high Z _n thermoelectric material is important.

As such a thermoelectric material,
(1) Bi-Te-based, Pb-Te-based, Si-Ge-based compound semiconductors,
(2) Co-based oxide ceramics such as Na _x CoO ₂ (0.3 ≦ x ≦ 0.8), (ZnO) _m In ₂ O ₃ (1 ≦ m ≦ 19), Ca ₃ Co ₄ O ₉ ,
(3) a skutterudite compound such as Zn—Sb, Co—Sb, or Fe—Sb,
(4) Half-Heusler compounds such as ZrNiSn,
(5) Silicides such as FeSi ₂ and MnSi _x (6) A cage-like cluster lattice mainly composed of group IV elements and Ba—Ga—Ge, Ba—Ga— in which atoms are incorporated into the internal space Clathrate compounds such as Si and Ba-Ga-Sn,
Etc. are known.

  Among these, clathrate compounds are inexpensive raw materials, do not contain elements with a large environmental load, and exhibit high thermoelectric properties in the middle temperature range around 700 ° C. Therefore, various proposals have heretofore been made regarding the composition and manufacturing method of thermoelectric materials composed of various clathrate compounds.

For example, Patent Document 1 discloses that
(A) Silicon clathrate 46, which is a mixed lattice of Si ₂₀ clusters consisting of dodecahedrons of Si atoms and Si ₂₄ clusters consisting of 14-sided Si atoms, or Si ₂₈ consisting of 16-sided Si ₂₀ clusters and Si atoms In the silicon clathrate 34 which is a mixed lattice with the cluster, a clathrate compound in which Ba is included in the silicon cluster and a part of the Si atoms constituting the silicon cluster lattice is substituted with P, and
(B) A raw material formulated so as to obtain a clathrate compound having a target composition is subjected to a mechanical alloying treatment and then subjected to a heat treatment, and the resulting pre-sintered product is pulverized to discharge the pulverized powder. Manufacturing method of thermoelectric element for plasma sintering,
Is disclosed.
In the same document,
(1) The inclusion of Ba having a mass larger than that of Si in the internal space of the silicon cluster causes the lattice vibration to be scattered and the thermal conductivity to be reduced,
(2) Inclusion of divalent Ba in the internal space of the silicon cluster improves electrical conductivity, and
(3) By substituting part of the Si atoms constituting the silicon cluster lattice with P, the clathrate compound that is metallic due to the inclusion of Ba becomes an n-type semiconductor, and the Seebeck coefficient is improved.
Is described.

In addition, in Patent Document 2,
(A) a clathrate compound in which, in the silicon clathrate 46 or the silicon clathrate 34, Ba is encapsulated inside the silicon cluster, and a part of Si atoms constituting the silicon cluster lattice is substituted with Al; and
(B) Thermoelectric material that is mechanically alloyed to obtain a clathrate compound having a target composition, pre-heated and re-pulverized the resulting mixed powder, and discharge powder sintering the pulverized powder. Device manufacturing method,
Is disclosed.
In the same document,
(1) The inclusion of Ba having a mass larger than that of Si in the internal space of the silicon cluster causes the lattice vibration to be scattered and the thermal conductivity to be reduced,
(2) Inclusion of divalent Ba in the internal space of the silicon cluster improves electrical conductivity, and
(3) By substituting a part of Si atoms constituting the silicon cluster lattice with Al, the clathrate compound that is metallic due to the inclusion of Ba becomes a p-type semiconductor, and the Seebeck coefficient is improved.
Is described.

Non-Patent Document 1 discloses an n-type Ba ₈ Ga ₁₆ Ge ₃₀ [Ba ₈ (Ga, Ge) ₄₆ ] clathrate crystal produced by the Czochralski method.
This document describes that the dimensionless figure of merit ZT at 900 K of a disk cut out from the obtained crystal is 1.35.

Patent Document 3 discloses that
(A) a thermoelectric material comprising a clathrate compound having a composition represented by Ba ₈ Ga _x Ge _44-x (14 ≦ x ≦ 18), and
(B) a method for producing a thermoelectric material in which raw materials formulated so as to become a clathrate compound having a target composition are melted, cast and pulverized, and the pulverized powder is subjected to discharge plasma sintering;
Is disclosed.
This document describes that the formation of a segregation phase is suppressed by adopting an atomic ratio of 44 host lattice atoms (Ga + Ge) atoms to 8 guest atoms (Ba) atoms.

Patent Document 4 discloses that
(A) a thermoelectric material comprising a clathrate compound having a composition represented by Ba ₈ Ga _x Si _46-x (14 ≦ x ≦ 24), and
(B) A method for producing a thermoelectric material in which a raw material blended so as to obtain a clathrate compound having a target composition is arc-melted and pulverized and sintered by discharge plasma,
Is disclosed.
This document describes that a clathrate compound having such a composition exhibits excellent characteristics as an n-type thermoelectric material.

Patent Document 5 discloses two phases of Ba ₂₄ Ga ₉ Ge ₉₁ (type III clathrate compound) 80 vol% -Ba ₈ Ga ₁₂ Ge ₃₄ (type I clathrate compound) 20 vol% produced by the Czochralski method. A thermoelectric material comprising a mixed tissue is disclosed.
In the same document,
(1) When the volume fraction of a type III compound having a low electrical resistivity is large, even if a type I compound having a high electrical resistivity is mixed, the electrical resistivity hardly changes.
(2) By using a two-phase mixed structure, it becomes impossible for heat-conducting phonons to propagate to other phases, and the thermal conductivity is greatly reduced, and
(3) With such a composition, the dimensionless figure of merit ZT at 650 ° C. is 1.65,
Is described.

Further, Non-Patent Document 2 discloses a method for producing a Ba ₈ Ga _x Ge _46-x (3 ≦ x ≦ 18) n-type thermoelectric material by arc melting.
In the same document,
(1) There is a solid solution limit of Ga in the vicinity of x = 16,
(2) In 4.5 ≦ x ≦ 16, as the Ga concentration increases, the number of electron carriers decreases, so that the electrical resistivity and Seebeck coefficient increase, and
(3) The increase in Ga increases the cluster lattice composed of a tetrahedron, thereby increasing the Ba rattling and lowering the lattice thermal conductivity.
Is described.
Non-Patent Document 3 discloses a method for producing single crystals of Sr ₈ Ga ₁₆ Ge ₃₀ and Ba ₈ Ga ₁₆ Ge ₃₀ by the Ga self-flux method.
In this document, when Ba (Sr): Ga: Ge = 8: 38: 30 is weighed and synthesized by the self-flux method, Ba ₈ Ga ₁₆ Ge ₃₀ is p-type and Sr ₈ Ga ₁₆ Ge ₃₀ is n-type thermoelectric. The point which becomes material is indicated.
Further, Non-Patent Document 4 discloses a method for producing Ba ₈ Ga ₁₆ Ge ₃₀ by arc melting → melting / solid phase reaction → discharge plasma sintering.

JP 2001-048517 A JP 2001-044519 A JP 2006-253291 A Japanese Patent Laid-Open No. 2005-170764 JP 2006-344833 A A. Saramat et al., J. Appl. Phys. 99 (2006) 023708 N.L.Okamoto et al., J.Appl.Phys. 100 (2006) 073504 K. Umeo et al., J. Phys. Soc. Jpn., 74 (2005) 2145 Shunsuke Mitshiro, Proceedings of the Annual Conference of the Thermoelectric Society of Japan (2006) 98

Thermoelectric materials containing Ba and / or Ga (for example, the clathrate compounds described above) have inherently high thermoelectric properties. However, the material obtained by the melting / casting method has a problem that the dimensionless figure of merit is low because there are many pores.
In the case of a thermoelectric material containing Ga, a single crystal can also be obtained by a Ga self-flux method. However, since the self-flux method is a method of crystal growth naturally, the size of the obtained crystal changes from production to production. Therefore, a sample having a sufficient cross-sectional area for thermoelectric generation cannot be obtained stably.
Furthermore, a single crystal can also be produced using the Czochralski method. However, the single crystal produced by the Czochralski method has a concentration variation in the growth direction. Therefore, there is a problem that a uniform material cannot be obtained, and the figure of merit changes depending on the location to be cut out.

  On the other hand, a method in which a thermoelectric material having a predetermined composition is synthesized and then pulverized and sintered has an advantage that a material having a small composition and a uniform composition can be obtained. However, Ba has a high vapor pressure and Ga has a low melting point. Therefore, when a thermoelectric material containing Ba and / or Ga is produced by a sintering method, there is a problem that Ba and / or Ga tends to flow out during heating and composition control is difficult. In particular, in a thermoelectric material in which Ba or Ga serves as a carrier dopant, the outflow of Ba or Ga during manufacture causes a decrease in carriers and a decrease in electrical conductivity.

A problem to be solved by the present invention is a method for producing a thermoelectric material capable of suppressing the outflow of Ba and / or Ga and precisely controlling the composition when producing a thermoelectric material containing Ba and / or Ga. It is to provide.
In addition, another problem to be solved by the present invention is that when a thermoelectric material in which Ba or Ga is a carrier dopant is produced, it is possible to suppress a decrease in electrical conductivity due to outflow of Ba or Ga. It is providing the manufacturing method of a thermoelectric material.

In order to solve the above problems, a method for producing a thermoelectric material according to the present invention includes:
A thermoelectric material having at least one of Ba and Ga as a constituent element, and an absolute value of Seebeck coefficient S at room temperature of 20 μV / K or more, or a main member made of a precursor that becomes the thermoelectric material by solid-phase reaction;
Coexisting with an auxiliary member made of a compound containing at least one of the constituent elements,
The gist of the invention is that it includes a heat treatment step for heat treating the main member.

When heating a main member made of a thermoelectric material containing Ba and / or Ga or a precursor thereof at a predetermined temperature, an auxiliary member made of a compound containing Ba and / or Ga around the main member (for example, a heating jig) If a sheet for wrapping the raw material, a powder bed packed around a molded body or a sintered body, etc.) are allowed to coexist, the outflow of Ba and / or Ga can be suppressed. Therefore, precise control of the composition becomes possible.
In particular, when this method is applied to a thermoelectric material in which Ba or Ga is a carrier dopant, it is possible to suppress a decrease in electrical conductivity due to these outflows, and high thermoelectric characteristics can be obtained.

Hereinafter, an embodiment of the present invention will be described in detail.
[1. Method for manufacturing thermoelectric material]
The manufacturing method of the thermoelectric material according to the present invention is as follows:
A thermoelectric material having at least one of Ba and Ga as a constituent element, and an absolute value of Seebeck coefficient S at room temperature of 20 μV / K or more, or a main member made of a precursor that becomes the thermoelectric material by solid-phase reaction;
Coexisting with an auxiliary member made of a compound containing at least one of the constituent elements,
A heat treatment step of heat treating the main member;

[1.1 Main members]
[1.1.1 Composition of main member]
The main member is made of a thermoelectric material or a precursor thereof.
In the present invention, “thermoelectric material” refers to a material having at least one of Ba and Ga as a constituent element and an absolute value of Seebeck coefficient S at room temperature of 20 μV / K or more.
A “precursor” refers to a thermoelectric material that satisfies the above-described conditions by a solid-phase reaction, and specifically corresponds to a raw material mixture formulated so as to obtain a target composition.
The main member may include one kind of thermoelectric material or a precursor for obtaining the same, or may include a mixture of two or more kinds of thermoelectric materials or a precursor for obtaining the same.
Furthermore, the main member may include only the thermoelectric material or the precursor, or may include components other than the thermoelectric material and the precursor. Examples of components other than the thermoelectric material and the precursor include a sintering aid and inevitable impurities. The smaller the other components that cause the deterioration of thermoelectric properties, the better.

As a thermoelectric material that satisfies the above conditions, for example,
(1) a clathrate compound containing Ba and / or Ga,
_{(2) BaB 6, La 2} -x Ba x CuO 4, YBa 2 Cu 3 O 7, BaSi 2, Ba y Fe x Co 4-x Sb 12, Ba y Ni x Co 4-x Sb 12, FeGa 3, RuGa ₃ , OsGa ₃ , CuGaO ₂ , GaP, GaAs, GaSb, GaN,
(3) a complex comprising two or more of the compounds of (1) and (2) above,
and so on.
Among these, the clathrate compound is particularly suitable as a thermoelectric material to which the present invention is applied because it exhibits high electrical conductivity, high Seebeck coefficient, and low thermal conductivity.

  Specific examples of the clathrate compound containing Ba and / or Ga and having an absolute value of Seebeck coefficient S of 20 μV / K or more include the following. The thermoelectric material may be composed of any one of these clathrate compounds, or may be a mixture of two or more.

A first specific example of the clathrate compound is constituted by the following formula (1). (1) The clathrate compound represented by the formula has a type I or type VIII crystal structure.
_{Ba 8-x A x B 46} -yz Ga y C z ··· (1)
However,
A is one or more elements selected from Group IA elements, Group IIA elements, and Group IIIA elements,
B is one or more elements selected from group IVB elements,
C is one or more elements selected from transition metal elements, IIB group elements, IIIB group elements, and VB group elements.
0 ≦ x ≦ 8, 0 ≦ y ≦ 46, 0 ≦ z ≦ 46, y + z ≦ 46, (8−x) + y> 0.

In the formula (1), “A” represents a guest atom included in the internal space of the bowl-shaped cluster lattice, similarly to Ba. A acts to scatter phonons and lower the thermal conductivity κ.
A is a group IA element ( ₃ Li, ₁₁ Na, ₁₉ K, ₃₇ Rb, ₅₅ Cs, ₈₇ Fr), a group IIA element ( ₄ Be, ₁₂ Mg, ₂₀ Ca, ₃₈ Sr, ₅₆ Ba, ₈₈ Ra), or And IIIA group elements ( ₂₁ Sc, ₃₉ Y, ₅₇ La to ₇₁ Lu, ₈₉ Ac to ₁₀₃ Lr). A may be any one of these elements, or two or more.

“B” represents a host atom constituting a cage cluster lattice, similarly to Ga.
B consists of IVB group elements _{(6 C, 14 Si, 32} Ge, 50 Sn, 82 Pb). B may be any one of these elements, or two or more.

“C” represents an element that substitutes all or part of the host atom. C has an effect of adjusting the carrier to an optimum concentration.
C is a transition metal element, IIB group elements _{(30 Zn, 48 Cd, 80} Hg), IIIB group elements _{(5 B, 13 Al, 31} Ga, 49 In, 81 Tl), or, VB group elements ₍₇ N, ₁₅ P, ₃₃ As, ₅₁ Sb, ₈₃ Bi). C may be any one of these elements, or two or more.

“X” represents the number of elements A contained as guest atoms. “Y” represents the number of elements B contained as host atoms, and “z” represents the number of elements C substituting all or part of the host atoms. Furthermore, “(8−x) + y> 0” indicates that at least one of Ba and Ga is included in the constituent elements.
The general composition of a clathrate compound of type I structure or type VIII structure is guest atom: host atom = 8: 46. The guest atom may be Ba alone, or all or part of Ba may be substituted with the element A. Similarly, the host atom may be only Ga, or all or part of Ga may be substituted with the element B and / or the element C.

Regarding x, 0 ≦ x ≦ 2 is preferable. When x is 2 or less, there are advantages that carrier control can be performed by A, carrier mobility is not greatly reduced, and a phonon scattering source is obtained and thermal conductivity is reduced.
As for z, 0 ≦ z ≦ 2 is preferable. When z is 2 or less, there are advantages that carrier control can be performed by C, carrier mobility is not greatly reduced, and a phonon scattering source is obtained, and thermal conductivity is reduced.
Regarding y, 14 ≦ y ≦ 24 is preferable. If 14 ≦ y ≦ 24, the Seebeck coefficient is increased and the thermal conductivity is reduced, so that there is an advantage that the figure of merit is high. y is more preferably 16 ≦ y ≦ 24. When y is 16 or more, the outflow of Ga is remarkable in the conventional method, but there is an advantage that the outflow of Ga from the thermoelectric material can be prevented by using this manufacturing method.

A second specific example of the clathrate compound is composed of the following formula (2). The clathrate compound represented by the formula (2) has a type III crystal structure.
_{Ba 24-x A x B 100} -yz Ga y C z ··· (2)
However,
A is one or more elements selected from Group IA elements, Group IIA elements, and Group IIIA elements,
B is one or more elements selected from group IVB elements,
C is one or more elements selected from transition metal elements, IIB group elements, IIIB group elements, and VB group elements.
0 ≦ x ≦ 24, 0 ≦ y ≦ 100, 0 ≦ z ≦ 100, y + z ≦ 100, (24−x) + y> 0.

In the formula (2), “x” represents the number of elements A contained as guest atoms. “Y” represents the number of elements B contained as host atoms, and “z” represents the number of elements C substituting all or part of the host atoms. Furthermore, “(24−x) + y> 0” indicates that at least one of Ba and Ga is included in the constituent elements.
The general composition of the clathrate compound of type III structure is guest atom: host atom = 24: 100. The guest atom may be Ba alone, or all or part of Ba may be substituted with the element A. The host atom may be Ga alone, or all or part of Ga may be substituted with the element B and / or the element C.
Since the other points regarding the expression (2) are the same as the expression (1), the description thereof is omitted.

[1.1.2 Shape of main member]
In manufacturing a thermoelectric element, a main member made of a thermoelectric material or a precursor thereof is heated in various steps. The present invention can be applied to any process.
Therefore, the main member is
(A) a powder made of a thermoelectric material, a molded body thereof or a sintered body thereof, or
(B) a powder comprising a precursor or a molded body thereof,
Either may be sufficient.

When manufacturing a thermoelectric element by a sintering method, the powder used for sintering can be manufactured by various methods.
As a method for producing a powder made of a thermoelectric material,
(1) A melting / casting method in which raw materials blended at a predetermined ratio are melted and cast, and the ingot is crushed.
(2) A solid phase reaction method in which a raw material (precursor) blended at a predetermined ratio is heated at a temperature below the melting point of the raw material to cause a solid phase reaction and pulverize the reaction product,
(3) A single crystal method for producing a single crystal from raw materials blended at a predetermined ratio and crushing the single crystal,
and so on.

Among these, the single crystal method has an advantage that a thermoelectric element having high thermoelectric characteristics can be easily obtained because a heterogeneous phase is hardly generated.
For example, in the case of a thermoelectric material containing Ga, the single crystal is preferably manufactured using a Ga self-flux method. The Ga self-flux method refers to a method of growing crystals in the presence of excess Ga (solid-liquid coexistence region). The single crystal is pulverized to an appropriate particle size. The obtained powder is sintered as it is or after being formed into a molded body.

[1.2 Auxiliary members]
[1.2.1 Composition of auxiliary member]
The auxiliary member is made of a compound containing at least one of the constituent elements (Ba and Ga) of the thermoelectric material. For example, when the thermoelectric material contains only Ba as a constituent element among Ba and Ga, a compound containing Ba is used for the auxiliary member. For example, when the thermoelectric material contains only Ga as a constituent element among Ba and Ga, a compound containing Ga is used for the auxiliary member. Further, when the thermoelectric material contains both Ba and Ga as constituent elements, a compound containing either one or both of Ba and Ga is used for the auxiliary member.

There are various compounds as the compound containing Ba and / or Ga. The compound constituting the auxiliary member is particularly preferably a compound represented by the formula (3).
_{Ba 1-x A x Ga 4} -y B y ··· (3)
However,
A is one or more elements selected from Group IA elements, Group IIA elements, and Group IIIA elements,
B is one or more elements selected from transition metal elements, IIB group elements, IIIB group elements, IVB group elements, and VB group elements.
0 ≦ x ≦ 1, 0 ≦ y ≦ 4, (1-x) + (4-y)> 0.

(3) where "A" represents an element that replaces all or part of the Ba of the intermetallic compound BaGa _4.
A is a group IA element ( ₃ Li, ₁₁ Na, ₁₉ K, ₃₇ Rb, ₅₅ Cs, ₈₇ Fr), a group IIA element ( ₄ Be, ₁₂ Mg, ₂₀ Ca, ₃₈ Sr, ₅₆ Ba, ₈₈ Ra), or And IIIA group elements ( ₂₁ Sc, ₃₉ Y, ₅₇ La to ₇₁ Lu, ₈₉ Ac to ₁₀₃ Lr). A may be any one of these elements, or two or more.

“B” represents an element that substitutes all or part of Ga in the intermetallic compound BaGa ₄ .
Of B, a transition metal element, IIB group elements _{(30 Zn, 48 Cd, 80} Hg), IIIB group elements _{(5 B, 13 Al, 31} Ga, 49 In, 81 Tl), IVB group elements ₍₆ C, ₁₄ Si , ₃₂ Ge, ₅₀ Sn, ₈₂ Pb), or a VB group element ( ₇ N, ₁₅ P, ₃₃ As, ₅₁ Sb, ₈₃ Bi). B may be any one of these elements, or two or more.

“X” represents the number of elements A substituting Ba, and “y” represents the number of elements B substituting Ga. Further, “(1-x) + (4-y)> 0” represents that the constituent element contains at least one of Ba or Ga.
When the thermoelectric material or its precursor contains a dopant or a substitution element other than Ba and Ga, the auxiliary member may contain elements A and B other than Ba and Ga. In order to suppress the outflow of Ba and / or Ga during heating, the auxiliary member is particularly preferably BaGa ₄ . BaGa ₄ has the highest melting point among the Ba—Ga based compounds and also has a high concentration of Ba and Ga, so that the outflow of Ba and / or Ga from the main member can be efficiently suppressed.

Other specific examples of compounds that can be used as auxiliary members include, for example,
(1) BaGa ₂ , BaSi ₂ , BaGe, BaGe ₂ , Ba ₂ Sn, CaGa ₂ , SrGa ₂ , AGa ₂ (A is a group IIIA element),
(2) Doping material of the compound of (1) above,
and so on.

[1.2.2 Shape of auxiliary member]
The auxiliary member is disposed so as to surround the main member in order to suppress the outflow of Ba and / or Ga from the main member. The shape of the auxiliary member is not particularly limited, and an auxiliary member having an optimal shape is used according to the shape, composition, heat treatment purpose, and the like of the main member.
As an auxiliary member, specifically,
(1) A container for containing a molded body, a heating jig such as a die and a punch used for pressure sintering such as hot pressing and SPS sintering,
(2) A sheet for wrapping a powder, a molded body or a sintered body at the time of degreasing, homogenization heat treatment, solid phase reaction, atmospheric pressure sintering, pressure sintering,
(3) A powder bed filled around a powder, a molded body or a sintered body at the time of degreasing, homogenization heat treatment, solid phase reaction, atmospheric pressure sintering, pressure sintering,
and so on.

Various heating jigs can be manufactured by, for example, sintering powder made of a compound containing Ba and / or Ga and processing the powder into a predetermined shape.
Moreover, a sheet | seat can be manufactured by rolling a molten material, for example.
Moreover, the powder used for a powder bed can be manufactured by, for example, synthesizing a compound containing Ba and / or Ga by a dissolution / casting method, a solid phase reaction method, or the like, and pulverizing the compound to an appropriate particle size.

[1.3 Heat treatment]
The heat treatment is performed in a state where the main member and the auxiliary member coexist. The main member is heat-treated for various purposes.
As heat treatment of the main member, for example,
(1) Heat treatment for degreasing a powder compact made of a thermoelectric material;
(2) heat treatment for pressureless sintering of a powder compact made of a thermoelectric material;
(3) heat treatment for pressure sintering (eg, hot pressing, SPS sintering, etc.) of a powder or molded body made of a thermoelectric material;
(4) Heat treatment for homogenizing the components of the powder or sintered body made of the thermoelectric material,
(5) A heat treatment for synthesizing a thermoelectric material having a target composition by subjecting a precursor powder or a molded body to a solid phase reaction,
and so on.
The heat treatment condition is not particularly limited, and an optimum condition may be selected according to the purpose of heat treatment of the main member.

[2. Operation of Thermoelectric Material Manufacturing Method]
A sintered body made of a thermoelectric material is generally manufactured by filling a carbon jig with a powder of a thermoelectric material and sintering it. However, if the thermoelectric material contains Ga with a low melting point and / or Ba with a high vapor pressure, these escape from the material during sintering. When Ba or Ga escapes from the thermoelectric material, defects are generated or a composition deviation from the weighed value occurs. In particular, in a p-type thermoelectric material in which Ba or Ga serves as a hole dopant, if these flow out during sintering, hole carriers are reduced and electrical conductivity is lowered. On the other hand, when Ba or Ga flows out in the n-type thermoelectric material, not only the composition control becomes difficult, but these flow out causes an excess of electron carriers, resulting in deterioration of thermoelectric characteristics. The opposite is also true, that is, when Ba or Ge is an electron dopant.

On the other hand, when a main member made of a thermoelectric material containing Ba and / or Ga or a precursor thereof is heated at a predetermined temperature, an auxiliary member made of a compound containing Ba and / or Ga around the main member (for example, , A heating jig, a sheet for wrapping the raw material, a powder bed packed around a molded body or a sintered body, etc.), the Ba concentration and / or Ga concentration around the main member increases, and the main member during heat treatment Outflow of Ba and / or Ga from can be suppressed. Therefore, precise control of the composition becomes possible.
In particular, when this method is applied to a p-type thermoelectric material in which Ba or Ga is a hole dopant, it is possible to prevent a decrease in hole carriers due to these outflows. Therefore, a thermoelectric material having a higher electrical conductivity and output factor than the conventional method can be obtained. Further, when this method is applied to an n-type thermoelectric material, not only composition control is facilitated, but also an excessive increase in electron carriers due to outflow of Ba or Ga can be suppressed. The same can be done when Ba and Ga are electron dopants. Therefore, a thermoelectric material having a higher Seebeck coefficient and a figure of merit than the conventional method can be obtained.

(Examples 1-2, Comparative Examples 1-3)
[1. Preparation of sample]
In order to show the effect of spreading BaGa ₄ around the clathrate compound as a powder bed during sintering, a single crystal was synthesized by the Ga self-flux method, and the powder obtained by pulverizing the single crystal was used as a starting material. A sintered body was produced. FIG. 1 shows the synthesis procedure.

[1.1 Synthesis of clathrate compound by Ga self-flux method]
First, a clathrate compound single crystal was synthesized using a Ga self-flux method. That is, the raw materials were weighed so that Ba: Ga: Ge = 8: 40: x in the glove box. x was 27 or 28. The weighed raw material was vacuum sealed in a quartz tube. Further, after holding the quartz tube at 1050 ° C. for 5 hours, the temperature was lowered to 750 ° C. at 3 ° C./h. The synthesized clathrate compound single crystal was pulverized to obtain a powder.

[1.2 Preparation of sintered body (Examples 1 and 2)]
The clathrate compound powder synthesized under the condition of x = 27 (Example 1) or x = 28 (Example 2) was compacted into pellets of φ12 mm at 50 MPa. The pellet was filled in a sintering jig, and BaGa ₄ powder was spread around the pellet. Furthermore, the pellet was sintered by SPS sintering. Sintering was performed by (1) pressurizing the pellet to 50 MPa, (2) raising the temperature to 800 ° C., and holding for 30 minutes.

[1.3 Production of sintered body (Comparative Examples 1 to 3)]
The powder of clathrate compound synthesized under the condition of x = 27 (Comparative Example 1) or x = 28 (Comparative Example 2) was filled in the sintering jig as it was. Next, the powder was sintered by hot pressing. Sintering was performed by (1) pressurizing the powder to 50 MPa and (2) raising the temperature to 900 ° C. and holding it for 5 hours.
Further, the clathrate compound powder synthesized under the condition of x = 28 (Comparative Example 3) was filled in the sintering jig as it was. Next, the powder was sintered by SPS sintering. Sintering was performed by (1) pressurizing the powder to 50 MPa and (2) raising the temperature to 900 ° C. and holding it for 1 hour.

[2. Test method]
SEM / EDX observation was performed about the obtained sintered compact. Moreover, the rod-shaped sample was cut out from the obtained sintered compact, and the thermoelectric characteristic was evaluated.

[3. result]
FIG. 2 shows a cross-sectional photograph, a backscattered electron image, and an EDX element mapping of the sintered body obtained in Example 2. From the SEM observation, no trace of the effluent exuded from the clathrate compound toward BaGa ₄ was observed. Furthermore, the concentration distribution of Ba, Ga, and Ge showed a clear difference at the boundary between the clathrate compound and BaGa ₄ . Therefore, it is considered that the diffusion and reaction of Ba, Ga, Ge from the clathrate compound to BaGa ₄ is small.

In FIG. 3, the temperature dependence of the electrical conductivity (sigma) of the sintered compact obtained in Examples 1-2 and Comparative Examples 1-3 is shown. FIG. 4 shows the temperature dependence of the output factor PF of these sintered bodies.
In the sintered bodies obtained in Examples 1 and 2, the electrical conductivity σ at 300 to 500 ° C. was improved as compared with Comparative Examples 1 to 3 synthesized with the same weighing values. In addition, the increase in electrical conductivity σ was the main cause, and the output factor PF also increased. This is presumably because the outflow of Ga, which is a hole dopant, and the decrease in hole carriers due to this were suppressed by laying a powder bed (BaGa ₄ ) around the clathrate compound.

(Example 3, Comparative Example 4)
[1. Preparation of sample]
In order to show the effect of spreading BaGa ₄ as a powder bed around the clathrate compound during sintering, a powder obtained by synthesizing a polycrystal by an arc melting method and pulverizing the polycrystal is used as a starting material, A ligature was prepared. FIG. 5 shows the synthesis procedure.

[1.1 Synthesis of clathrate compound by arc melting method]
In the glove box, the raw materials were weighed so that Ba: Ga: Ge: Zn = 8.2: 18: 27.5: 0.5. The weighed raw material was arc melted for 3 minutes. Arc melting was repeated three or more times in order to eliminate unmelted material and homogenize it. The synthesized clathrate compound was pulverized to obtain a powder.

[1.2 Production of sintered body (Example 3)]
The synthesized clathrate compound powder was compacted into pellets of φ12 mm at 50 MPa. The pellet was filled in a sintering jig, and BaGa ₄ powder was spread around the pellet. Furthermore, the pellet was sintered by SPS sintering. The sintering conditions were pressure: 50 MPa, temperature: 750 ° C., holding time: 30 minutes.

[1.3 Production of sintered body (Comparative Example 4)]
The synthesized clathrate compound powder was filled in a sintering jig as it was. Next, the powder was sintered by SPS sintering. The sintering conditions were pressure: 50 MPa, temperature: 800 ° C., holding time: 1 hour.

[2. Test method and results]
A rod-like sample was cut out from the obtained sintered body, and the thermoelectric characteristics were evaluated. FIG. 6 shows the temperature dependence of the electrical conductivity σ of the sintered bodies obtained in Example 3 and Comparative Example 4. The sintered body obtained in Example 3 has an improved electrical conductivity σ at 300 to 500 ° C. as compared with Comparative Example 4 synthesized with the same weighing value. This is presumably because the outflow of Ga, which is a hole dopant, and the decrease in hole carriers due to this were suppressed by laying a powder bed (BaGa ₄ ) around the clathrate compound.

Example 4
The formation energy (FE) was calculated by the electronic structure calculation program VASP (plane wave-PAW method using density functional theory), and the stability of Ga substitution in the clathrate compound was evaluated. GGA was used for the exchange correlation potential. The definition of formation energy (FE) is defined by F.E. E. (AB → AC) = E (AC) − [E (AB) + E (C) −E (B)]. E is the total energy; E. The lower the value, the easier it is to synthesize the substance AC from the substance AB.

FIG. 7 shows the formation energy (FE) of GaGe ₄₅ with respect to the Ge ₄₆ cluster lattice. 8 shows the formation energy (FE) of BaGa ₂ Ge ₄₄ with respect to the Ge ₄₆ cluster lattice.
Type I clathrate compounds belong to the space group Pm-3n, and in Ba ₈ Ga _y Ge _46-y , Ga substituted for Ge sites is a group of three sites 6c, 16i, and 24k classified by symmetry. Place in one. A lower formation energy (FE) means a more stable crystal structure. Since the formation energy (FE) of GaGe ₄₅ shows a positive value, it is considered that the Ge ₄₆ cluster lattice is stable and becomes unstable when Ga is substituted. Further, the formation energy (FE) of BaGe ₄₆ containing one Ba is -1.1 eV, while the formation energy (FE) of BaGa ₂ Ge ₄₄ is -1.5 to -2. Since the voltage is 1 eV, the inclusion of Ba and the substitution of Ga are simultaneously performed to stabilize the system. Therefore, it is considered that preventing Ba from flowing out has an effect of preventing Ga from flowing out. From the electronic calculation results of FIGS. 7 and 8, in Ba ₈ Ga _y Ge _46-y , Ga is arranged in the order of 6c, 24k, and 16i sites so that Ga is not adjacent and does not form a Ga—Ga bond. It is thought to go.
When y> 16, Ga is adjacent to each other at any Ga position, and a Ga—Ga bond must be formed. Therefore, in the vicinity of y = 16, F.I. E. Increases rapidly as x increases. It is considered that the crystal structure rapidly becomes unstable near y = 16, and when y ≧ 16, the outflow of Ga from the material becomes remarkable.
Therefore, in particular, when the present invention is applied to a clathrate compound with y ≧ 16, Ga outflow during sintering is suppressed, and a decrease in electrical conductivity σ can be suppressed.

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the gist of the present invention.

  The method for producing a thermoelectric material according to the present invention includes a solar thermoelectric generator, a seawater temperature difference thermoelectric generator, a fossil fuel thermoelectric generator, various thermoelectric generators such as a regenerative generator for factory exhaust heat and automobile exhaust heat, and a light detection element. Configures thermoelectric elements used in laser diodes, field effect transistors, photomultiplier tubes, spectrophotometer cells, precision temperature control devices such as chromatographic columns, thermostats, air conditioners, refrigerators, clock power supplies, etc. It can be used as a method for producing a thermoelectric material.

It is a flowchart which shows the manufacturing method of the sintered compact using the single crystal synthesize | combined by Ga self-flux method as a starting material. It is a cross-sectional photograph, a backscattered electron image, and EDX element mapping of the sintered compact obtained in Example 2. It is a figure which shows the temperature dependence of electrical conductivity (sigma) of the sintered compact obtained by Examples 1-2 and Comparative Examples 1-3. It is a figure which shows the temperature dependence of the output factor PF of the sintered compact obtained in Examples 1-2 and Comparative Examples 1-3. It is a flowchart which shows the manufacturing method of the sintered compact using the polycrystal synthesize | combined by the arc melting method as a starting material. It is a figure which shows the temperature dependence of electrical conductivity (sigma) of the sintered compact obtained in Example 3 and Comparative Example 4. FIG. For Ge ₄₆ clusters grating is a diagram showing a formation energy of GaGe ₄₅ (F.E.). Ge ₄₆ is a diagram showing a formation energy (F.E.) of the BaGa ₂ Ge ₄₄ for the cluster lattice.

Claims (8)

A thermoelectric material having at least one of Ba and Ga as a constituent element, and an absolute value of Seebeck coefficient S at room temperature of 20 μV / K or more, or a main member made of a precursor that becomes the thermoelectric material by solid-phase reaction;
Coexisting with an auxiliary member made of a compound containing at least one of the constituent elements,
A method for producing a thermoelectric material, comprising a heat treatment step for heat treating the main member. The main member is
(A) a powder made of the thermoelectric material, a molded body or a sintered body thereof, or
(B) The method for producing a thermoelectric material according to claim 1, which is a powder comprising the precursor or a molded body thereof.   The main member is a powder obtained by pulverizing a single crystal made of the thermoelectric material that contains Ga as the constituent element and is manufactured by a Ga self-flux method, a molded body thereof, or a sintered body thereof. Item 3. A method for producing a thermoelectric material according to Item 1 or 2.   The method of manufacturing a thermoelectric material according to any one of claims 1 to 3, wherein the auxiliary member is a heating jig, a sheet, or a powder bed surrounding the main member. The said thermoelectric material is a manufacturing method of the thermoelectric material as described in any one of Claim 1 to 4 containing the clathrate compound represented by following (1) Formula.
_{Ba 8-x A x B 46} -yz Ga y C z ··· (1)
However,
A is one or more elements selected from Group IA elements, Group IIA elements, and Group IIIA elements,
B is one or more elements selected from group IVB elements,
C is one or more elements selected from transition metal elements, IIB group elements, IIIB group elements, and VB group elements.
0 ≦ x ≦ 8, 0 ≦ y ≦ 46, 0 ≦ z ≦ 46, y + z ≦ 46, (8−x) + y> 0. The said thermoelectric material is a manufacturing method of the thermoelectric material as described in any one of Claim 1-5 containing the clathrate compound represented by following (2) Formula.
_{Ba 24-x A x B 100} -yz Ga y C z ··· (2)
However,
A is one or more elements selected from Group IA elements, Group IIA elements, and Group IIIA elements,
B is one or more elements selected from group IVB elements,
C is one or more elements selected from transition metal elements, IIB group elements, IIIB group elements, and VB group elements.
0 ≦ x ≦ 24, 0 ≦ y ≦ 100, 0 ≦ z ≦ 100, y + z ≦ 100, (24−x) + y> 0. The said compound which comprises the said auxiliary member is a manufacturing method of the thermoelectric material as described in any one of Claim 1 to 6 which has a composition represented by following (3) Formula.
_{Ba 1-x A x Ga 4} -y B y ··· (3)
However,
A is one or more elements selected from Group IA elements, Group IIA elements, and Group IIIA elements,
B is one or more elements selected from transition metal elements, IIB group elements, IIIB group elements, IVB group elements, and VB group elements.
0 ≦ x ≦ 1, 0 ≦ y ≦ 4, (1-x) + (4-y)> 0. Said compounds, method for producing a thermoelectric material according to any one of claims 1 is a BaGa ₄ to 6 constituting the auxiliary member.

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